The present work devotes to studying the electric properties: electric quadrupole moment and electric field gradient (EFG) of molecules PdH and (PdH)^-2 based on the full relativistic theory. It is the first time to explore that the force constants are essentially caused by electric field gradients, and indirectly with spectroscopic data. If EFG is positive, zero or negative, then the will be positive, zero or negative. Therefore, second order force constants are adjustable to changing the intensity of EFG.